At the beginning of beginning

FE Team | Published: October 06, 2007 00:00:00 | Updated: February 01, 2018 00:00:00

Syed Fattahul Alim
Secret of the early universe is still a subject of great controversy and debate among the scientists. That the universe as we see it started from a singularity through the Big Bang is now a well-established theory. But the nitty-gritty of how that dimensionless point of singularity led to the enormous latter-day universe with its complicated structure of stellar systems interspersed with fathomless voids filled with what has, of late, been learnt to be dark matter and dark energy is still a mystery. Scientists working on the fringe of physical science are toying with very novel ideas. Some of them are an admixture of old and new concepts. Physicists and astrophysicists are trying to resolve many unsolved questions with the help of these new concepts and theories.
The age-old gravitation may again come to the aid of astrophysics to retrace the sequence of events within a split second that led to the inflationary phase that expanded primordial universe 10 to the power 80 times. The sudden change or acceleration during the formation of early universe must have sent out gravitational waves all around. Scientists are expecting that even 15 billion years after that great event, the telltale signatures of that early gravitational waves can still be detected. Scientists have already installed different kinds of devices to detect those earliest waves of gravitation.
In the following Craig J Hogan, professor of physics and astronomy at the University of Washington, Seattle explains the hard scientific theories and experiments behind this daring attempts at unlocking the secrets of early universe.
'Our view of the universe is about to change forever. Since science began, all our knowledge of what lies above, below and around us has come from long-familiar forms of energy: light, produced by distant astrophysical objects; and matter, in the form of particles such as cosmic rays. But we are now in a position to study the universe using an entirely different form of energy that until now has never been directly detected - gravitational waves.
A key prediction of Einstein's general theory of relativity, gravitational waves are vibrations of space-time generated by the acceleration of all forms of mass and energy. Extreme gravitational environments such as black holes or neutron-star binaries generate waves with the largest amplitudes, while the frequency of the waves depends on how such sources move. Small-scale motions, such as those of stellar-mass black holes, generate high-frequency gravitational waves, while larger objects, such as massive black holes, move more slowly and produce lower-frequency signals. Passing through material of any kind at the speed of light, gravitational waves fill the entire universe and may therefore carry information from the beginning of space-time itself.
Around the world several gravitational-wave detectors are currently taking data, in the hope that they will detect these tiny disturbances of space-time directly for the first time. These large interferometers - LIGO in the US, GEO-600 in Germany, VIRGO in Italy and TAMA in Japan - are all looking for minute changes in the relative lengths of two kilometre-scale arms induced by a passing gravitational wave. In the next few years they should be able to detect the high-frequency signals (roughly 100 Hz or more) produced by the most extreme gravitational objects
Gravitational-wave detectors are not restricted to the Earth: an international project called the Laser Interferometer Space Antenna (LISA) is currently awaiting critical funding decisions that could see it launched in about 2017. Away from the noisy environment of our planet, LISA's three spacecraft will use lasers to form a trio of interferometer arms each five million kilometres long. The mission will therefore be able to detect disturbances in space-time down to 1 mHz and below, probing a region of the gravitational-wave spectrum that is known to contain a large number and variety of sources.
Since gravitational waves allow us to study the universe with a new form of energy that couples to everything, gravitational-wave detectors may also lead to totally unexpected discoveries - as did the telescope and the microscope in their times. Moreover, gravitational waves provide a detailed record of events that took place in the first second or so of the universe, which should allow us to constrain models such as cosmic inflation and other extreme and uncharted physics of the early universe. Indeed, these ghostly disturbances of space-time effectively turn the early universe into a sophisticated laboratory for ultrahigh-energy physics that could help tackle the problem of quantum gravity.
Probing inflation
Gravity has already revealed to us an invisible universe. About 70 years ago, Fritz Zwicky discovered the gravitational effects of what we now call dark matter, when he realized that the speed with which certain galaxies move could not be explained by the amount of visible matter. Determining the nature of dark matter alone - which is now thought to make up about 21per cent of the universe - is one of the great challenges of modern physics. Furthermore, about 10 years ago astronomers found that an even larger fraction of the universe (about 75 per cent) is made up of "dark energy" - a gravitationally repulsive substance that is causing the expansion of the universe to accelerate. Can we even begin to guess what we might find when we use gravity itself to probe the universe?
While we are likely to discover unforeseen sources of gravitational waves in the recent (i.e. nearby) universe, one great hope is that gravitational-wave detectors will tell us about the extreme gravitational conditions that existed much earlier in the universe's history. Electromagnetic radiation has already provided direct evidence of many processes that took place in this era. For example, the spectra from distant matter has indirectly shed light on how light nuclei were produced in the first few minutes of the universe, while the cosmic microwave background provides a snapshot of the universe as it was 380 000 years after the Big Bang.
This background - a cold sea of low-frequency electromagnetic radiation - was produced after the universe had expanded and cooled sufficiently to allow hydrogen atoms to form (a process called recombination). Photons that had previously been scattered by charged particles in the primordial plasma could now propagate freely - their observed wavelength today having been stretched to the microwave region. The cosmic microwave background has told us much about the propagation of acoustic waves in the primordial plasma, among other important results such as the geometry of space. But gravitational waves can tell us much more about the early universe by probing motion that occurred at such early times and on such small scales that its electromagnetic traces have long since been washed out in thermal equilibrium.
The idea that currently generates the most excitement is the possibility of detecting gravitational waves from cosmic inflation, a period of accelerated expansion that began immediately after the Big Bang during which the volume of the universe increased by a factor of up to 10 exp 80 in a tiny fraction of a second. Inflation is the best model we have for explaining the large-scale structure of the universe - in other words, for making the universe big, for kick-starting cosmic expansion and for producing the fluctuations in space-time that seeded galaxy formation. Yet we know very little about the physics of this extremely brief chapter of cosmic history.
As gravitational waves were produced by motion on the quantum scale during inflation, detecting them would indicate the existence of gravitons - the hypothetical particles of gravity and hence space-time itself. These single quanta are thought to have imprinted small fluctuations onto the fabric of space-time that were blown up to enormous scales by inflation. Detecting such primordial gravitational waves would therefore test whether quantum mechanics is correct under very high densities. It would also allow cosmologists to estimate parameters such as the rate of inflationary expansion, which are currently poorly constrained.
Primordial waves
The best way to search for these early gravitational waves is to study the cosmic microwave background (CMB) radiation. Thanks to many beautiful results from experiments such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP), we have already detected the imprint of other quanta in the CMB - work for which COBE researchers John Mather and George Smoot were awarded the 2006 Nobel Prize for Physics. Small differences in the temperature of the microwave background in different patches of the sky are direct evidence for fluctuations in the "inflaton field" that drove inflation. These fluctuations are the reason why matter clumped together to create the galaxies and other cosmic structure that we see today. With even more heroic experimental efforts it may be possible to tease out the much weaker signature of gravitons in the microwave background.
In order to separate the contributions of the graviton- and inflaton-induced fluctuations, we need to study the polarization of the CMB. These photons became polarized - i.e. the electric-field component of the electromagnetic wave tends to point in a particular direction - when they scattered off free electrons either during recombination or a few hundred million years later when the first stars formed and reionized the surrounding gas. Because the pattern of temperature fluctuations in the CMB has a "quadrupolar" component - that is, it is brighter along some axes than along others - the electrons are made to jiggle more along certain directions. This gives rise to different polarizations, but the quadrupolar signal generated by gravitational waves is special because gravitational waves themselves have a purely quadrupolar character. (It is this property that allows us to detect gravitational waves using interferometers, see figure 3.) As a result, gravitational waves can generate polarization fluctuations even in places where there is no local temperature perturbation.
In 2001 researchers working on the DASI experiment in Antarctica detected polarization fluctuations in the CMB at the level expected - i.e. a few per cent of the temperature fluctuations - and since then other experiments including WMAP, BOOMERANG and the Cosmic Background Imager have verified and extended the DASI results (see figure 4). But disentangling the particular polarization signal expected only from gravitons - which involves a vortex-like pattern of polarization - is much harder because its contribution is so small. Detecting the polarization pattern at fainter levels is the target of new experiments such as the Robinson Gravitational Wave Background Telescope ("BICEP") in Antarctica and of a variety of planned and proposed experiments both on the ground and in space, including the European Space Agency's Planck Explorer and, eventually, NASA's Beyond Einstein.